talk to a sheet metal fabricator with a cnc turret punch
if they have the od punche, they can punch/or nibble out the rectangular hole and then knock out a clean round blank with the hole already in it. make you 20 of them for the same price as one.

or saw and file to fit.

get Andy's yet to be finished tome on filing. (go andy)

the Gene Olson
Sculptor
Elk River, MN

Robert,

I wouldn't bother with the compass cutter, myself. Since you only need one of these, I think the most straightforward way to do it is with a jeweler's saw and files.

Start by cleaning your brass with some pumice and water, followed by soapy water and a rinse. Dry it thoroughly and apply a thin coat of Dychem layout fluid or Magic Marker. This will allow you to easily see scribed lines.

Next, determine the center of your 35mm circle and make a small center punch mark with a sharp scribe point. Using a machinist's dividers with very sharp points, scribe your circle, being careful to make only one thin line. Using a steel ruler and a scribe or a needle, similarly layout your window. Drill a hole in the waste area of the window.

Using a #0 or #1 blade in a jeweler's saw frame, saw out the window first. You want to be sure your piece of brass is small enough that the saw frame can pass all around it, so you can cut the window easily. When sawing, try to cut just at the edge of your scribed line. It sometines helps to use a magnifier headset to see better, and have plenty of light. WOuldn't hurt to practice on some scrap untile you feel comfortable with the saw, either. A bit of beeswax or paraffin on the blade as a lubricant helps, too.

For sawing with a jeweler's saw, it is very handy to have a good support for the work. A piece of 1/2" thick wood about 2" wide with a "V" notch cut about 1" into the end of it can be clamped or screwed to your benchtop and will allow you to hold the work sceurely and support the work on both sides of the saw cut.

After the window is cut, file it as needed for final finish, and then cut out the circle. File it and you're done.

The whole operation takes about the same amount of time to do as it does to write down the instructions, for me at least. I saw much better than I type. :-)

If I was doing this myself, I would probably use a 2/0 or 3/0 blade, as Brad would. I've sawed hundreds, quite probably thousands of feet of cut with a jeweler's saw, so I feel very comfortable with blades as fine as 6/0. For someone unfamiliar with sawing, I think a 3/0 might, as Brad says, give you a lot of practice changing blades. A 6/0 might not even make it to the cut - you'd likely break it just putting it in the saw frame. I'm always amazed that a blade almost as fine as a human hair can be made with sharp, alternately set teeth and sell as cheaply as they do.

Brad's point about not getting water on saw blades is really spot on. I live in the tropics, and I have to keep mine oiled for storage or the salt air rusts them away. I too buy them by the gross, since I have to mail order, and since I do mostly blacksmithing these days, I don't go through them like I used to.